This invention relates to passive data displays, and particularly to methods and means for controlling and monitoring the operation of a passive electro-optical data display, such as a liquid crystal display, that shows numbers or symbols in response to selective activation of separately controllable line segments.
Various types of electro-optical data displays have been available commercially for a number of years and are used widely for display of optical information such as alpha-numeric data and the like. One widely used type of data display is a liquid crystal display which, in a typical case, utilizes seven segments which are selectively energized to display each digit. As is well known, a liquid crystal cell is composed of two glass plates covered with transparent conductive layers on the surface facing one another. The plates sandwich a nematic liquid crystal. A voltage applied to the thin film electrodes formed by the transparent conductive layers produces an electric field which results in zones of different refraction coefficients. Depending upon the structure of the transparent electrodes, the liquid crystal cell can be used for the display of numbers, letters, or other data. Depending on how it is utilized, the liquid crystal display can be constructed to operate in a reflection or transmission mode on the basis of whether the impinging or transient light is to be dispersed.
Passive displays such as liquid crystal cells basically differ from active displays in that they do not produce their own light but only modulate external light. Among various advantages, liquid crystal cells have the special benefit that their current consumption is very low. However, this very low current drain does introduce a disadvantage in that a test of the operation of the electrodes forming the line of the characters becomes virtually impossible.
In active displays, such as LED displays, a check of the display data is simple. As the individual segment currents are measured, they are retroactively compared with the data input. If necessary, an error signal is derived therefrom. A segment check of this type does not lead to useful results with passive displays, such as liquid crystal displays. This is due in part to the very low and relatively strongly scattered segment currents. To increase the reliability of the symbol display, the possibility of redundant operation has been considered. This is disclosed in German Offenlegungschrift No. 24 02 749. Alternatively, attempts have been made to maintain the display by further segmentation of the individual symbols. See German Offenlegungschrift No. 24 59 488. However, these measures result in significant production costs and expensive circuitry.
To overcome such unsatisfactory conditions in liquid crystal displays, proposals have been made in German Offenlegungschrift No. 27 43 907 that the front plate carry additional electrodes (check electrodes) which are electrically connected to one of the segment forming electrodes ("endangered segment electrodes"). Upon failure or occurrence of improper control signals, an alarm signal is to be displayed. The back plate is to contain a "check back electrode" which is located only opposite the check electrodes. However, this means providing an additional check electrode for each of the so-called endangered segment electrodes, i.e., at least five segment electrodes in a seven segment number display. Also, the back plate of the display must contain an additional "check back electrode" which interacts only with the check electrodes. Thus, aside from the seven segment electrodes and the conventionally used back electrodes, the arrangement must include five check electrodes and a check back electrode for the purpose of the proposed test cycle The entire arrangement must fit on a carrier plate. This is quite complicated. The very large number of separate electrical connections results in a corresponding substantial source of contact errors. Check electrodes represent expensive, special equipment. Hence, displays of the conventional kind do not carry display check arrangements.
Generally, in analyzing the principal possible sources of errors and their effects on a display system of the aforementioned type, it is possible to localize the actual sources of error to particular places. These can be found specifically in the area of the display element or, electrically ahead of this element in the area of the display control. A few sources of error can be partially attributed to production defects, or to aging effects to which each component is subjected. These produce visible effects on the symbol display. For example, aging, an unsealed cell, operation above permitted temperature ranges, broken glass, destruction of polarizers, and the like directly affect all the segments or the entire display component. Such phenomena at the display itself always affect all the display elements. Hence, an error is easily visible.
Errors which are not visible, on the other hand, create incorrect displays which cannot be recognized visibly by an observer. Such an inaccurate display is usually caused by a defect in the control when individual segments fail or are incorrectly activated during the display operation. Such errors may arise, for example, from a defective integrated circuit in the control unit, defective printed conductors, faulty contacts at the display component, and especially failure of individual control lines, etc. The aforementioned sources of error are particularly likely to cause optically imperceptible display errors. Hence, it is essential that officially licensed or otherwise authorized devices which can be calibrated, such as, for example, taxi meters, money counters, scales and meters, and the like be monitored by test devices so their inaccuracies can be disclosed.
An object of the invention is to improve data displays.
Another object of the invention is to avoid the aforementioned disadvantages.
Still another object of the invention is to provide a simple method and means for controlling, monitoring, and checking the operation of an essentially passive data display, particularly a liquid crystal data display in order to make optically invisible display errors perceptible.